The invention relates to a matching circuit, by which a radio-frequency amplifier can electrically be adjusted to a load, and a method for adapting the amplifier to load impedance at various amplifier output power levels by means of the matching circuit.
A radio channel changes constantly as a function of time. For instance, as a subscriber terminal moves, changes in multipath propagation environment cause changes in the radio channel. It is important to adapt the subscriber terminal""s transmission power level such that the signal level is sufficient for a base station to receive and detect the signal, and that it causes as little interference as possible to other users of the system. Transmitters use power amplifiers for adjusting the transmission power to a desired level. Receivers use pre-amplifiers for amplifying the power of the received signal that has become weak on a radio channel. Output powers of the amplifiers in the subscriber terminals and the base station are adjusted as the radio channel changes. In addition, the amplifiers have to be matched with an antenna and operating environment of the antenna by impedance matching. Conventionally, the matching circuit of a transmitter amplifier consists of passive circuit elements with constant values. A matching circuit of this kind can be adapted in optimal state only to one predetermined output power level, for which is generally selected the maximum power level. In addition, if the transmitter is supposed to operate on any desired channel of a broad band, the optimal adaption setting changes according to the channel.
Harmonic signals, of which the second harmonic signal, in particular, is harmful in radio-frequency applications, and which result from amplifier non-linearities, reduce the efficiency of the amplifier. To improve the efficiency, a typical solution is a frequency selective circuit, which is tuned to a desired harmonic signal frequency. It can be implemented by a simple LC circuit (inductance-capacitance), which resonates at the tuned frequency and thus short-circuits the signal portion that comprises the harmonic frequency. The prior art implementation has a problem that in order to operate efficiently the circuit must have a high Q value, i.e. quality factor, and as the Q value is high the circuit operates optimally only on a narrow frequency band.
U.S. Pat. No. 5,276,912 discloses a radio-frequency power amplifier having variable output power. The publication sets forth a plurality of different alternatives to match load impedance with the output power. The alternatives of the solution employ e.g. switches consisting of PIN diodes or transistors, or capacitors controlled by bias voltage, or varactors. However, the disclosed solutions have disadvantages: for instance, components used cause interference, such as distortion, to the output signal. In addition, when the device switches on, sudden, major changes in power generate major voltage variations, and consequently sensitive components must be protected by various solutions. Neither does the publication provide a solution for the problem of reduced amplifier efficiency resulting from the harmonic signals.
Microelectromechanical MEMS (microelectromechanical system) components have currently been developed. In general, the microelectromechanical (MEMS) components are manufactured onto a semiconductor substrate, such as silicon (Si) or gallium-arsenide (GaAs), because in this manner it is possible to integrate them with conventional semi-conductor components as manufacturing technology advances. It is also possible to manufacture microelectromechanical (MEMS) components onto a dielectric material, such as aluminium oxide. The MEMS components have a structure that is, at least in part, off from the base material, typically a membrane-like bridge, which opens and closes the circuit of the component. The bridge is controlled to on/off states by control voltage. The control voltage is applied to the semi-conductor substrate to a conductive layer obtained by precipitation, which layer forms one or more electrodes. The electrode is located below the bridge, and therefore the position of the bridge can be controlled: when the bridge touches the electrode, the circuit is closed, and when the bridge does not touch the electrode, the circuit is open. If there is an isolating layer on top of the electrode, the bridge has no galvanic contact to the electrode. Thus at least, direct current does not provide a closed circuit. Capacitance between the bridge and the electrode is then high. Components manufactured by MEMS technology have mainly been used in write heads of ink-jet printers and in sensors, such as triggering mechanisms of air bags in vehicles. Radio frequency applications mainly utilize passive components manufactured by MEMS technology because of space saving. dr
The object of the invention is to provide a method and equipment implementing the method such that a radio-frequency amplifier can be adjusted to an optimal state at a plurality of different power levels and operating frequencies. This is achieved with a matching circuit for adapting the amplifier to load impedance at various output power levels of the amplifier, the matching circuit comprising an interface for receiving control signals, which determine capacitance of one or more capacitors. In the matching circuit of the invention at least one matching circuit capacitor is a microelectromechanical (MEMS) capacitor, whose capacitance the control signals adjust to make the amplifier operate optimally at a selected frequency at a power level used, and consequently making the amplifier operate optimally in the whole desired power range.
The invention also relates to a matching circuit for adapting the amplifier to load impedance at various output power levels of the amplifier, the matching circuit comprising one or more LC circuits, i.e. an electric circuit switching consisting of at least one coil and at least one capacitor for tuning harmonic signals resulting from amplifier non-linearities. In the matching circuit of the invention, at least one LC circuit capacitor is an adjustable microelectromechanical (MEMS) capacitor, the LC circuit comprises an interface for receiving the control signals adjusting the capacitance of the microelectromechanical (MEMS) capacitor, the control signal adjusts the capacitance of the microelectromechanical (MEMS) capacitor such that the LC circuit resonates at the frequency of each harmonic signal to be tuned.
The invention also relates to a method for adapting an amplifier to load impedance with a matching circuit at various output power levels of the amplifier, the matching circuit comprising an interface for receiving control signals which determine capacitance of one or more capacitors. The method of the invention adjusts the capacitance of at least one microelectromechanical (MEMS) capacitor of the matching circuit such that the amplifier operates optimally in a broad output power range.
The invention also relates to a method for adjusting the amplifier to load impedance with a matching circuit at various output power levels of the amplifier, the matching circuit comprising one or more LC circuits, i.e. an electric circuit switching consisting of at least one coil and at least one capacitor for tuning harmonic signals resulting from amplifier non-linearities. In the method of the invention, harmonic signals are tuned with an LC circuit, the LC circuit comprising at least one adjustable microelectromechanical (MEMS) capacitor, control signals adjusting the capacitance of the microelectromechanical (MEMS) capacitor through an interface in the LC circuit are received, the capacitance of the microelectromechanical (MEMS) capacitor is adjusted by at least one control signal such that the LC circuit resonates at the frequency of the harmonic signal to be tuned.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea that adjustable, microelectromechanical (MEMS) capacitors are introduced to radio-frequency amplifiers and/or LC circuits connected thereto intended for tuning harmonic signals. In this way, it is possible to optimize the efficiency in energy transmission from the amplifier to the load impedance as the output power and operating frequency of the amplifier varies.
Several advantages are achieved by the method and the system of the invention. By the method and system according to a first embodiment of the invention for adjusting a matching circuit of a radio-frequency amplifier it is possible to adapt the optimum load impedance of the amplifier to the load impedance according to changes in the output power and operating frequency. Thanks to adjustability, the method and system of a second embodiment of the invention for tuning harmonic signals allow to achieve a more accurate and quicker adaption to the harmonic frequency concerned.
The MEMS capacitors are extremely linear as to their electric properties, because they have no P/N interface (interface of positive and negative charge carriers), and therefore they do not generate non-linearities that are typical to semi-conductor components, such as PIN diodes and FET (Field-Effect Transistor) components. Non-linearity of the component is harmful, because it causes signal distortion. In addition, the MEMS capacitors tolerate well large voltage variations and they have good AC voltage toleration, so there is less need for separate components or circuits protecting against voltage peaks. Furthermore, losses of the MEMS capacitors are very small and their service life as compared with MEMS switches is considerably long.